There are several types of prostheses available for upper extremity amputees including passive devices, body powered devices and myoelectric devices. Passive devices are primarily for cosmetic appearance with limited functional benefit. Body powered devices require the use of the shoulder and/or elbow to open and close a terminal device. There are many types of terminal devices that can be used and the use of these prostheses is widespread and favored in many specific circumstances. Myoelectric prostheses rely on the detection of a signal produced by muscle contraction in the residual limb to power the terminal device. These have several advantages compared with passive devices and body powered devices including lower energy requirements for use and proportional control. The advent of myoelectric prostheses has revolutionized the field of upper extremity prosthetics, but there are many persisting limitations.
The field of prosthetics has seen many recent advancements thus improving the quality of life for amputees. Despite this, these advanced prosthetics are limited by the human-prosthesis interface. For myoelectric prostheses, the mode of interaction for signaling the prosthesis has been via surface electromyography (EMG). For bidirectional control in a single plane, two EMG channels are needed. Because of the limited number of muscles available to serve as independent signals for the fingers, individual digital control with myoelectric prostheses has not been possible for patients with partial or total hand amputations. Having an appropriate number of muscles available, ideally with the same desired function, is a requisite for successful myoelectric prosthetic use. The lack of available muscle signals has limited the functional ability for patients with partial hand amputations, total hand amputations, below elbow amputations, above elbow amputation, and shoulder disarticulations.
Currently there are limited control options for individuals that have undergone a partial hand amputation. The mechanical fingers of a myoelectric hand are capable of independent finger flexion and extension only if the requisite muscle signals are available. The first myoelectric prosthesis was designed by Reiter in 1948 and provided finger flexion and extension by use of a single muscle. Clinically this is not preferred since independent and intuitive control is not provided due to the lack of available EMG signals from appropriate muscle groups. The level of amputation determines available muscles for surface EMGs and in turn the ability to control multiple degrees of freedoms. A typical partial hand myoelectric fitting consists of two or three surface electrodes that capture EMG readings from either intrinsic hand muscles or from more proximal wrist and finger flexor and extensor muscle groups allowing control of hand opening and closure. While forearm musculature produces reliable signals, prosthetic control is not natural as the muscle contraction is not associated with the desired prosthetic finger motion.
For below elbow amputations at the level of the forearm, amputees have been limited in achieving all of the normal degrees of freedom possible for the wrist, forearm and hand function due to the limited EMG inputs available to control the desired degrees of freedom. Basic options such as grasp and release are possible, but control of functions such as radial deviation, ulnar deviation, wrist flexion, wrist extension, pronation and supination have not been possible due the limited number of EMG signals available. Independent control of the thumb and fingers has also not been possible due to inadequate signals available for detection.
In above elbow amputees including shoulder level disarticulations, targeted muscle reinnervation (TMR) has allowed an increase in the number of signals available. Nerves that previously controlled the forearm and hand, which are still present in the residual limb, can be transferred into a remaining muscle in the upper arm or shoulder to produce a new and unique signal for detection. For above elbow amputees transfers such as the median nerve into one head of the biceps, for example, allows one head of the biceps to control elbow flexion and the re-innervated head of the biceps to control grasp of the hand. Similarly, TMR can be performed using the radial nerve to re-innervate one head of the triceps to allow digital extension in addition to the previously possible elbow extension.
An advanced recent prosthetic design and technology known as pattern recognition allows a greater number of muscles to be detected for specific patterns of prosthetic use. This technique involves placing multiple surface electrodes on the skin and using a computer to analyze the pattern of muscle contracture for a given function. This pattern can then be used to direct myoelectric prosthetic control rather than relying on a direct individual muscle contracture for the function. This has been more widespread in above elbow amputees but is possible for below elbow amputees as well. The limitation of this technique is the inability to preform multiple planes of motion simultaneously and independently. The advantages of one muscle controlling one function includes the highly intuitive nature of prosthetic control, the ability to detect multiple EMG inputs that are each specifically linked to a particular function, and to allow these multiple functions to occur simultaneously in a coordinated and intuitive manner.
An additional limitation in optimal control of a myoelectric prosthetic is cross-talk. Cross-talk occurs when multiple muscles are producing multiple signals simultaneously thus making detection of one specific signal difficult. This unwanted detection of signals from muscles other than the target should be minimized when possible with surgery. The previously described method of limiting cross-talk involves the use of adipofascial flaps that are placed between adjacent muscles.
The largest current limitation of myoelectric prostheses for upper extremity amputees centers on the need for a greater number of available myoelectric signals. Through the use of novel muscle transfers and nerve transfers, a much greater number of signals can be detected allowing a new generation of prostheses to be produced thus allowing a higher level of function for these patients.